It is estimated that it currently costs $1 million per kilogram to put a payload on the lunar surface. Further, limited power generating capability and the electrical load produced by onboard electronic circuitry result in operating issues that constrain the capability of a spacecraft of any given size and configuration. Therefore the cost, size, weight and capability of spacecraft components are always issues of concern. Frequently, trade offs must be made between cost, weight and size and capability.
In a spacecraft, command and control electronics are indispensible. Their complexity increases geometrically as demand for system capability grows. Command and control electronics monitor various conditions and components of a spacecraft using a plurality of analog and digital sensors/transducers. The command and control electronics are programmed to make a decision based in part on a sensor input and then cause the spacecraft to perform an action via a plurality of actuators that may be analog or digital devices.
Common digital interfaces include both serial busses and interfaces commonly referred to as “discrete” interfaces. Serial interfaces and discrete interfaces share the convention that the information that is conveyed has two states, which are commonly referred to as ones and zeros or binary information. Serial interfaces differ from discrete interfaces in that the information being conveyed by a discrete interface is usually only one bit; either a one or a zero. The single bit of information on a discrete interface may not change during a complete power cycle of the system it participates in or it may change frequently, but the changes are not generally made on any predetermined schedule. Serial data, on the other hand, conveys many bits of information by conveying a series of ones and zeroes at a predetermined frequency. Both interfaces are digital interfaces.
To determine the state (one or zero) of a signal applied to the input pins of a digital interface, the signal may be compared to a threshold voltage or a threshold current level. Comparator circuits of various constructions may be used for such a purpose. Some comparator circuits may receive an analog input and generate a digital output based on whether the analog input is greater than or less than the threshold value. A non-limiting example of such a comparator is a LM139 comparator circuit as described in LM139/LM239/LM339 A Quad of Independently Functioning Comparators, National Semiconductor Corporation, 2002, which is incorporated herein by reference in it entirety. Other comparator circuits may receive a digital input and generate a digital output based on whether the digital input is greater than or less than the threshold value.
Digital interfaces may be used for interfaces between a variety of electronic equipment that are at least partially digital. Examples of such equipment include flight control computers, launch abort systems, radios, sensor systems, star trackers and thrust vector controllers. Binary information often comes from switches that may be located throughout a spacecraft. Switches may be manually operated by astronauts or may include switches within various pieces of equipment.
Because of the growing number and complexity of digital electronics, it is desirable to minimize the size and weight of electronics that communicate digitally in order to reduce launch costs. In addition, it is desirable to minimize the electrical load generated by the electronics. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
Although discussed herein in the context of a spacecraft or satellite, the subject matter disclosed herein may be applied to other types of vehicles or other electronic systems. Any reference to spacecraft applications is merely exemplary and is not intended to limit the scope of the following disclosure in any way.